This invention relates to an apparatus and method for three dimensional (3-D) vision sensing or measurement and, in particular, to an apparatus and method for three dimensional vision sensing or measurement utilizing holographic scanning of the object or surface whose coordinates are to be measured or sensed.
U.S. patent application No. 697,796, filed and assigned to the same assignee hereof, discloses a 3-D measurement system in which the projector of the system utilizes a holographic scanning mechanism to transform the projected radiant energy beam of the projector into a collection of projected light planes. These light planes are directed onto the surface of the object to be measured and a camera mounted at an angle to the projected planes records the images of the intersections of the planes and the surface. These images can then be uniquely interpreted or processed to provide the 3-D coordinates of every resolvable point in the camera image of the surface via the triangulation relationship of the known baseline distance between the camera and projector, and the known angles of the optical axes of the projector and camera relative to the baseline.
Holographic scanning mechanisms of the type to be used in the measurement system of the '796 application are commercially available and generally are formed as rotatably mounted glass disks having sectors each provided with grating grooves etched into the surface of the disk. When a radiant energy beam, such as, for example, a laser beam is projected at an angle through the disk, the beam is deflected by the gratings of the disk. The amount of deflection is dependent upon the grating spacing, so that by incorporating several sectors having different grating spacings about the disk, the beam can be deflected at different angles as the beam impinges on each of the sectors of the disk. Moreover, as the beam encounters a particular sector and is deflected through the unique deflection angle corresponding to the sector, the rotation of the sector causes the beam to scan a straight line. The overall effect of rotating the disk on the projected beam is thus to create a collection of projected planes.
Advantageously, because of the optical properties of the grating sectors of the holographic disk, the deflection angle, which is a diffraction phenomenon, is maintained even if the disk wobbles. Off center rotation errors also do not affect performance, so that by using the holographic disk, a lower cost measurement system can be realized for a given precision requirement. Furthermore, the ability of the holographic disk to provide a deflection change on a segment to segment basis in any preselectable manner is advantageous over the continuous scanning of a beam brought about by scanning mechanisms formed from continously rotated reflective surfaces or the like.
The utilization of a scanning beam as provided by the holographic disk to provide radiant energy along a plane, moreover, results in a more uniform intensity of the radiant energy as compared to spreading the energy over a plane with a lens. This provides a decided advantage when trying to maximize the dynamic range of the surface reflectivities over which a measurement and vision sensing system will operate.
In addition to disclosing the above discussed holographic measuring and vision sensing system, the '796 application also discloses that the number of projection planes realizable by such a system can be increased by cascading a plurality of scanning mechanisms, e.g., a plurality of holographic disks or by inserting a refracting material into the path of the radiant energy beam after passage through the scanning mechanism. In the latter case, the refractor provides a lateral offset to the beam as compared to the beam without the refractor and, therefore, effectively doubles the number of radiant energy planes realizable by the system.
U.S. Pat. No. 4,238,147, also assigned to same assignee hereof, discloses a 3-D measurement system in which the lens plane and film plane of the camera of the system and the plane of the radiant energy intersect in a common line. This type of configuration allows the camera focus to be set so that it is in proper focus for substantially all points of the surface to be recorded, without having to readjust the setting of the focus for differently located points on the surface.
While the above prior art systems have been found usable, there is still a need to provide an overall 3-D measurement system employing holographic scanning which is less costly to manufacture and which does not suffer from temperature sensitivity usually attendant holographic scanners. Furthermore, it is important in such a system that synchronism be maintained between the system components including scanner and camera system and that there be provided means for increasing the projected planes in an inexpensive and simple way.
It is therefore a primary object of the present invention to provide a 3-D measuring system in which holographic scanning is utilized and in which the system components are coordinated and arranged to minimize manufacturing costs, to provide precision and repeatability in the deflection process, to allow for arbitrary deflection without a loss in performance and to provide simplicity in the overall system design.
It is a further object of the present invention to provide a 3-D measurement system of the aforesaid type in which a mechanism is included for synchronization of the system components.
It is yet a further object of the present invention to provide arrangements for increasing the projection plane capacity of such a 3-D measurement system, without adding significantly to the complexity of the system.